1. Field of the Invention
The present invention relates to a narrow titanium-containing wire, a production process thereof, a nanostructure and an electron-emitting device, and more particularly to a narrow wire that can be widely used as a functional material or structural material for electron devices, microdevices and the like. In particular, it can be used as a functional material for photoelectric transducers, photo-catalytic devices, electron-emitting materials, narrow wires for micromachines, narrow wires for quantum effect devices, and the like, a production process thereof, a nanostructure comprising the narrow wire, and an electron-emitting device using the nanostructure.
2. Related Background Art
Titanium and alloys thereof have heretofore been widely used as structural materials for aircraft, automobile, chemical equipment and the like because they are light-weight, strong and hard to corrode. Besides, titanium and alloys thereof are also in use as medical materials because they are harmless to human bodies.
Recently, in research related solar cells, decomposition of injurious materials, antibacterial action, etc., extensive use had been made of the photo-conductive properties, photocatalytic activity and the like of titanium oxide.
Besides, the application range of titanium materials extends to many fields such as vacuum getter materials, electron-emitting materials, metallic alloys for hydrogen storage and electrodes for various electron devices.
On the other hand, thin films, narrow wires, small dots and the like of metals and semiconductors may exhibit specific electrical, optical and/or chemical properties in some cases because the movement of electrons is restricted to certain shorter characteristic lengths.
From this point of view, an interest in materials (nanostructures) having a structure smaller than 100 nm as functional materials is greatly increasing.
An example of a method for producing a nanostructure includes a production by semiconductor processing techniques including minute pattern writing techniques such as photolithography, electron beam exposure and x-ray diffraction exposure.
Aside from such a production method, it has been attempted to realize a novel nanostructure on the basis of a naturally formed regular structure, i.e., self-ordered structure. Since this technique leads to a possibility of producing a fine and special structure superior to those made by the conventional methods, many researchers are beginning to use it.
An example of the specific self-ordered nanostructure is an anodically oxidized aluminum film [see, for example, R. C. Furneaux, W. R. Rigby & A. P. Davidson, NATURE Vol. 337, p. 147 (1989)]. This anodically oxidized aluminum film (hereinafter called “porous alumina”) is formed by anodically oxidizing an Al plate in an acid electrolyte. As illustrated in FIG. 6, its feature resides in that it has a specific geometric structure that narrow cylindrical pores (nanoholes) 14, as extremely fine as several nanometers to several hundred nanometers in diameter, are arranged at intervals of several nanometers to several hundred nanometers parallel to each other. These narrow cylindrical pores 14 have a high aspect ratio and are excellent in linearity and uniformity of sectional diameter.
Various applications are being attempted by using the specific geometric structure of such a porous alumina as a base. The detailed explanation thereof is found in Masuda [Masuda, KOTAI-BUTSURI(Solid-State Physics), 31, 493, 1996]. Techniques for filling a metal or semiconductor into narrow pores and techniques for taking a replica are typical, and various applications including coloring, magnetic recording media, EL light-emitting devices, electrochromic devices, optical devices, solar cells and gas sensors have been attempted.
Further, applications to many fields, for example quantum effect devices such as quantum wires and MIM (metal-insulator-metal) tunnel effect devices, and molecular sensors using nanoholes as chemical reaction sites, are expected.
If such a nanostructure made with a highly functional material, i.e., titanium, is available, the nanostructure is expected to be utilized as a functional structure such as electron devices, microdevices, etc.
As an example where a nanostructure is produced by using a titanium material and controlling size and form, patterning of a thin film of the titanium material by semiconductor processing techniques including minute pattern writing techniques such as photolithography, electron beam exposure and x-ray diffraction exposure as described above may be mentioned. However, these techniques involve problems of poor yield and high cost of apparatus, and there is thus a demand for development of a simple method for producing a nanostructure with good reproducibility.
The method using the self-ordering phenomenon, particularly the method using the porous alumina as a base, is preferable to the method using a semiconductor processing technique because a nanostructure can be easily produced over a large area under good control.
As an example where a titanium-containing nanostructure was produced by applying such a method, an example by Masuda et al., in which porous TiO2 was formed by taking a replica of porous alumina with titanium oxide [Jpn. J. Appl. Phys., 31 L1775 -LI777(1992); and J. of Materials Sci. Lett., 15, 1228-1230(1996)] may be mentioned.
However, this method still has problems to be solved, such as it must go through many complicated steps in the process of taking the replica, and the crystallinity of TiO2 is poor since it is formed by electrodeposition.
On the other hand, it is often conducted to filling a metal or semiconductor into narrow pores of the porous alumina, thereby producing a nanostructure. Examples thereof include filling of Ni, Fe, Co. Cd or the like by an electrochemical method [see D. Al-Mawlawi et al., Mater. Res., 9,1014(1994); and Masuda et al., Hyomen-Gijutsu (Surface Techniques), Vol. 43, 798(1992)], and melt introduction of In, Sn, Se, Te or the like [see C. A. Huber et al., SCIENCE, 263, 800(1994)]. However, the filling of a Ti-containing material according to either method has not been reported for the reasons that the electrodeposition of Ti is not common, and that the Ti materials generally have a high melting point.
On the other hand, potassium titanate whiskers of the submicron size (0.2to 1.0 μm in diameter, 5 to 60 μm in length) have been developed as applications to fiber reinforced plastics, fiber reinforced metals and fiber reinforced ceramics [Nikon-Kinzoku-Gakkai-ski (Journal of The Japan Institute of Metals), 58, 69-77(1994)]. However, these materials are all powdery, and no technique for position-controlling and arranging them on a substrate is yet known. In order to expect specific electrical, optical and chemical properties as nanostructures, it is also necessary to further narrow the pores.